Hybrid stent

ABSTRACT

The invention is directed to an expandable stent for implantation in a body lumen, such as a coronary artery. The stent consists of radially expandable cylindrical rings generally aligned on a common axis and interconnected by one or more links. At least some of the links are formed of a polymer material providing longitudinal and flexural flexibility to the stent while maintaining sufficient column strength to space the rings along the longitudinal axis.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation application of parent application having U.S.Ser. No. 09/753,232 filed Dec. 28, 2000, now U.S. Pat. No. 6,565,599 thecontents of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

This invention relates to expandable endoprosthesis devices, generallycalled stents, which are adapted to be implanted into a patient's bodylumen, such as blood vessel, to maintain the patency thereof. Thesedevices are useful in the treatment of atherosclerotic stenosis in bloodvessels.

Stents are generally tubular-shaped devices which function to hold opena segment of a blood vessel, coronary artery, or other anatomical lumen.They are particularly suitable for use to support and hold back adissected arterial lining which can occlude the fluid passagewaytherethrough.

Further details of prior art stents can be found in U.S. Pat. No.3,868,956 (Alfidi et al.); U.S. Pat. No. 4,512,338 (Balko et al.); U.S.Pat. No. 4,553,545 (Maass et al.); U.S. Pat. No. 4,733,665 (Palmaz);U.S. Pat. No. 4,762,128 (Rosenbluth); U.S. Pat. No. 4,800,882(Gianturco); U.S. Pat. No. 4,856,516 (Hillstead); U.S. Pat. No.4,886,062 (Wiktor); U.S. Pat. No. 6,066,167 (Lau et al.); and U.S. Pat.No. B1 5,421,955 (Lau et al.), which are incorporated herein in theirentirety by reference thereto.

Various means have been described to deliver and implant stents. Onemethod frequently described for delivering a stent to a desiredintraluminal location includes mounting the expandable stent on anexpandable member, such as a balloon, provided on the distal end of anintravascular catheter, advancing the catheter to the desired locationwithin the patient's body lumen, inflating the balloon on the catheterto expand the stent into a permanent expanded condition and thendeflating the balloon and removing the catheter. One of the difficultiesencountered using prior stents involved maintaining the radial rigidityneeded to hold open a body lumen while at the same time maintaining thelongitudinal flexibility of the stent to facilitate its delivery. Oncethe stent is mounted on the balloon portion of the catheter, it is oftendelivered through tortuous vessels, including tortuous coronaryarteries. The stent must have numerous properties and characteristics,including a high degree of flexibility in order to appropriatelynavigate the tortuous coronary arteries. This flexibility must bebalanced against other features including radial strength once the stenthas been expanded and implanted in the artery. While other numerousprior art stents have had sufficient radial strength to hold open andmaintain the patency of a coronary artery, they have lacked theflexibility required to easily navigate tortuous vessels withoutdamaging the vessels during delivery.

Generally speaking, most prior art intravascular stents are formed froma metal such as stainless steel, which is balloon expandable andplastically deforms upon expansion to hold open a vessel. The componentparts of these types of stents typically are all formed of the same typeof metal, i.e., stainless steel. Other types of prior art stents may beformed from a polymer, again all of the component parts being formedfrom the same polymer material. These types of stents, the ones formedfrom a metal and the ones formed from a polymer, each have advantagesand disadvantages. One of the advantages of the metallic stents is theirhigh radial strength once expanded and implanted in the vessel. Adisadvantage may be that the metallic stent lacks flexibility which isimportant during the delivery of the stent to the target site. Withrespect to polymer stents, they may have a tendency to be quite flexibleand are advantageous for use during delivery through tortuous vessels,however, such polymer stents may lack the radial strength necessary toadequately support the lumen once implanted.

What has been needed and heretofore unavailable is a stent which has ahigh degree of flexibility so that it can be advanced through tortuouspassageways and can be readily expanded and yet have the mechanicalstrength to hold open the body lumen into which it expanded. The presentinvention satisfied this need.

SUMMARY OF THE INVENTION

The present invention is directed to an expandable stent which isrelatively flexible along its longitudinal axis to facilitate deliverythrough tortuous body lumens, but which is stiff and stable enoughradially in an expanded condition to maintain the patency of a bodylumen such as an artery when implanted therein.

The stent of the invention generally includes a plurality of radiallyexpandable cylindrical rings which are relatively independent in theirability to expand and to flex relative to one another. The individualradially expandable cylindrical rings of the stent are formed from ametallic material and are aligned on a common longitudinal axis. Theresulting stent structure is a series of radially expandable cylindricalrings which are spaced longitudinally close enough so that smalldissections in the wall of a body lumen may be pressed back intoposition against the lumenal wall, but not so close as to compromise thelongitudinal flexibility of the stent. The cylindrical rings areattached to each other by flexible links such that at least one flexiblelink attaches adjacent cylindrical rings. If desired, more than one linkcan be used to attach adjacent cylindrical rings. At least some of thelinks are formed from a polymeric material that provides flexibility tothe link and allows the stent to more easily bend or flex along itslongitudinal axis as the stent navigates through tortuous vessels orcoronary arteries. The flexibility of the links is balanced against thelinks having sufficient column strength to properly orient and separatethe cylindrical rings along the stent longitudinal axis so that therings do not telescope into each other or overlap one another. Thecombination of the flexible cylindrical rings and flexible linkscumulatively provides a stent which is flexible along its length andabout its longitudinal axis, yet is still relatively stiff in the radialdirection after it has been expanded in order to maintain the patency ofa vessel and to resist collapse.

One preferred structure for the expandable cylindrical rings which formthe stent of the present invention is generally a circumferentialundulating pattern, e.g., serpentine. The open reticulated structure ofthe stent allows for the perfusion of blood over a large portion of thearterial wall which can improve the healing and repair of a damagedarterial lining.

The stent embodying features of the invention can be readily deliveredto the desired body lumen, such as a coronary artery (peripheralvessels, bile ducts, etc.), by mounting the stent on an expandablemember of a delivery catheter, for example a balloon, and advancing thecatheter and stent assembly through the body lumen to the target site.Generally, the stent is compressed or crimped onto the balloon portionof the catheter so that the stent does not move longitudinally relativeto the balloon portion of the catheter during delivery through thearteries, and during expansion of the stent at the target site.

When the stent expanded, the radial expansion of the expandablecylindrical rings deforms the undulating or serpentine pattern similarto changes in a waveform which result from decreasing the waveform'samplitude and the frequency. The undulating patterns of the individualcylindrical rings can be in phase with each other or out of phase,depending on the stent design. The cylindrical rings of the stent areplastically deformed when expanded so that the stent will remain in theexpanded condition and therefore they must be sufficiently rigid whenexpanded to prevent the collapse thereof in use. During expansion of thestent, portions of the undulating pattern may tip outwardly resulting inprojecting members on the outer surface of the expanded stent. Theseprojecting members tip radially outwardly from the outer surface of thestent and embed into the vessel wall and help secure the expanded stentso that it does not move once it is implanted.

The links which interconnect adjacent cylindrical rings may have atransverse cross-section similar to the transverse dimensions of theundulating components of the expandable cylindrical rings. In oneembodiment, all of the links are joined at either the peaks or thevalleys of the undulating structure of the cylindrical rings. In thismanner there is little or no shortening of the stent upon expansion.

The number and location of links connecting the rings can be varied inorder to vary the desired longitudinal and flexural flexibility in thestent structure both in the unexpanded as well as the expandedcondition. These properties are important to minimize alteration of thenatural physiology of the body lumen into which the stent is implantedand to maintain the compliance of the body lumen which is internallysupported by the stent. Generally, the greater the longitudinal andflexural flexibility of the stent, the easier and the more safely it canbe delivered to the target site.

Other features and advantages of the present invention will become moreapparent from the following detailed description of the invention whentaken in conjunction with the accompanying exemplary drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational view, partially in section, of a stentembodying features of the invention which is mounted on a deliverycatheter and disposed within a damaged artery.

FIG. 2 is an elevational view, partially in section, similar to thatshown in FIG. 1 wherein the stent is expanded within a damaged artery.

FIG. 3 is an elevational view, partially in section, depicting theexpanded stent within the artery after withdrawal of the deliverycatheter.

FIG. 4 is a plan view of a flattened section of the stent of theinvention, illustrating the cylindrical rings attached by the links.

FIG. 5 is a plan view of a flattened section of a stent illustrating anundulating pattern in the expandable cylindrical rings of the stentwhich are out of phase.

FIG. 6 is a perspective view of the stent of FIG. 4 after it is fullyexpanded depicting some portions of the stent projecting radiallyoutwardly.

FIG. 7 is a plan view of a flattened section of a portion of twoadjacent rings attached by one of the links.

FIG. 8 is a plan view of a flattened section of one cylindrical ring ofthe stent and a link attached thereto.

FIG. 9 is a plan view of a flattened section of one cylindrical ringwith a link attached thereto.

FIG. 10 is a plan view of a flattened section of a portion of twocylindrical rings with a link attached thereto.

FIG. 11 is a plan view of a flattened section of a portion of onecylindrical ring with a link attached thereto.

FIG. 12 is a plan view of a flattened section of a portion of acylindrical ring with a link adhesively bonded thereto.

FIG. 13 is an elevational view of a portion of a link depicting analternative attachment method to attach one cylindrical ring to anadjacent cylindrical ring.

FIG. 14 is an elevational view of a portion of a link depicting analternative attachment method to attach one cylindrical ring to anadjacent cylindrical ring.

FIG. 15 is a perspective view of a mandrel having grooves for both thecylindrical rings and the links for use in the injection moldingprocess.

FIG. 16 is a perspective view of a quarter arc section of an outer moldcover having grooves for the cylindrical rings and links.

FIG. 17 is a perspective view of a quarter arc section of the outer moldcover having grooves for the cylindrical rings and links.

FIG. 18 is a partial perspective view of the mandrel with the quarterarc section outer mold covers positioned over the mandrel for use in theinjection molding process.

FIG. 19 is a partial elevational view of a portion of an outer moldcover depicting the gate through which the polymer is injected to formthe links.

FIG. 20 is a partial elevational view of a section of a cylindrical ringhaving a thin and thick portion and a polymer link encapsulating theapex portion of the ring.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a stent 10 incorporating features of the inventionwhich is mounted onto a delivery-catheter 11. The stent generallycomprises a plurality of radially expandable cylindrical rings 12disposed generally coaxially and interconnected by links 13 disposedbetween adjacent cylindrical elements. The delivery catheter 11 has anexpandable portion or balloon 14 for expanding of the stent 10 within anartery 15. The artery 15, as shown in FIG. 1 has an occluded portion ofthe arterial passageway that has been opened by a previous procedure,such as angioplasty.

The delivery catheter 11 onto which the stent 10 is mounted, isessentially the same as a conventional balloon dilatation catheter forangioplasty procedures. The balloon 14 may be formed of suitablematerials such as polyethylene, polyethylene terephthalate, polyvinylchloride, nylon and ionomers such as Surlyn® manufactured by the PolymerProducts Division of the Du Pont Company. Other polymers may also beused. In order for the stent 10 to remain in place on the balloon 14during delivery to the site of the damage within the artery 15, thestent 10 is crimped or compressed onto the balloon in a known manner.

Each radially expandable cylindrical ring 12 of the stent 10 may besubstantially independently expanded to some degree relative to adjacentrings. Therefore, the balloon 14 may be provided with an inflated shapeother than cylindrical, e.g., tapered, to facilitate implantation of thestent in a variety of body lumen shapes.

In one embodiment, the delivery of the stent 10 is accomplished in thefollowing manner. The stent is first mounted onto the inflatable balloon14 on the distal extremity of the delivery catheter by crimping orcompressing the stent in a known manner. The catheter-stent assembly isintroduced within the patient's vasculature in a conventional Seldingertechnique through a guiding catheter (not shown). A guide wire 18 isdisposed across the damaged arterial section and then the catheter-stentassembly is advanced over a guide wire 18 within the artery 15 until thestent is positioned at the target site 16. The balloon of the catheteris expanded, expanding the stent against the artery, which isillustrated in FIG. 2. While not shown in the drawing, the artery ispreferably expanded slightly by the expansion of the stent to seat orotherwise fix the stent to prevent movement. In some circumstancesduring the treatment of stenotic portions of an artery, the artery mayhave to be expanded considerably in order to facilitate passage of bloodor other fluid therethrough.

The stent 10 serves to hold open the artery 15 after the catheter 11 iswithdrawn, as illustrated by FIG. 3. Due to the formation of the stentfrom an elongated tubular member or a flat sheet, the undulatingcomponent of the cylindrical rings 12 of the stent is relatively flat intransverse cross-section, so that when the stent is expanded, thecylindrical rings are pressed into the wall of the artery and as aresult do not interfere with the blood flow through the artery. Thecylindrical elements 12 of stent which are pressed into the wall of theartery will eventually be covered with endothelial cell growth whichfurther minimizes blood flow interference. The undulating portion 17 ofthe cylindrical rings provide good tacking characteristics to preventstent movement within the artery. Furthermore, the closely spacedcylindrical rings at regular intervals provide uniform support for thewall of the artery 15, and consequently are well adapted to tack up andhold in place small flaps or dissections in the wall of the artery 15.

FIG. 4 is an enlarged plan view of the stent 10 shown in FIG. 1 with oneend of the stent shown in an exploded view to illustrate in greaterdetail the placement of links 13 between adjacent radially expandablecylindrical rings 12. Each of the links on one side of a cylindricalring is preferably placed to achieve maximum flexibility for a stent. Inthe embodiment shown in FIG. 4, the stent 10 has three links 13 betweenadjacent radially expandable cylindrical elements 12, which are spaced120° apart. Each of the links on one side of a cylindrical ring areoffset radially 60° from a corresponding link on the other side of thering. The alternating link pattern results in a stent havinglongitudinal and flexural flexibility in essentially all directions dueto the placement of the links. Various configurations for the placementof the links are possible, and several examples are illustratedschematically in FIGS. 4-5. However, as previously mentioned, all of thelinks of an individual stent should be secured to either the peaks orvalleys of the undulating structural portions 17 in order to helpprevent shortening of the stent during the expansion thereof.

FIG. 4 illustrates a stent of the present invention wherein three links13 are disposed between radially expandable cylindrical rings 12. Thelinks are distributed radially around the circumference of the stent ata 120° spacing. Disposing four or more links between adjacentcylindrical rings will generally give rise to the same considerationsdiscussed above for placement of one, two, and three links.

The properties of the stent 10 may also be varied by alteration of theundulating portions 17 of the cylindrical rings 12. FIG. 5 illustratesan alternative stent structure in which the cylindrical rings have anundulating shape so the undulations of one cylindrical ring 12 is out ofphase with adjacent cylindrical rings. The particular pattern and howmany undulations per unit of length around the circumference of thecylindrical rings, or the amplitude of the undulations, are chosen tofill particular mechanical requirements for the stent, such as radialstiffness.

With reference to FIG. 6, the cylindrical rings 12 are in the form ofundulation portions 17, as previously mentioned. The undulating portionis made up of a plurality of U-shaped members 31, W-shaped members 32,and Y-shaped members 33, each having a radius that more evenlydistributes expansion forces over the various members. After thecylindrical rings 12 have been radially expanded, outwardly projectingedges 34 are formed. That is, during radial expansion some of theU-shaped, W-shaped, or Y-shaped portions may tip radially outwardlythereby forming outwardly projecting edges. These outwardly projectingedges provide for a roughened outer wall surface of the stent 10 andassist in implanting the stent in the vascular wall by embedding intothe vascular wall. In other words, outwardly projecting edges embed intothe vascular wall, for example artery 15, as depicted in FIG. 3.Depending upon the dimensions of stent 10 and the thickness of thevarious members making up the serpentine pattern 30, any of the U-shapedmembers 31, W-shaped members 32, and Y-shaped members 33 can tipradially outwardly to form a projecting edge 34.

The stent patterns shown in FIGS. 1-6 are for illustration purposes onlyand can vary in shape and size to accommodate different vessels or bodylumens. Thus, rings connected by links can have any structural shapesand are not limited to the aforedescribed undulating rings, U-shaped,W-shaped, and Y-shaped portions, or to straight links connecting therings.

In keeping with the invention, the links 13 are formed from a flexiblepolymer material, or similar material, that is bendable and flexible toenhance longitudinal and flexural flexibility of the stent 10. Since thecylindrical rings 12 are independently formed out of a metal, such asstainless steel or the like, the rings must be connected together by thelinks. One aspect of the invention provides for various attachmentmechanisms for attaching the links to the rings.

As shown in FIG. 7, link 40 has a length which can vary, and connects aY portion 33 of one cylindrical ring (not shown) to the W-shaped portion32 of an adjacent cylindrical ring (not shown). The link has a first end41 and a second end 42 and a locking head 43 which is designed to fitwithin correspondingly shaped first cavity 44 and second cavity 45 asillustrated in FIG. 7. As will be further described herein, the polymerused to form the links is injection molded so that the locking heads cantake any particular form in order to fill the first and second cavities.An alternative link is shown in FIG. 8, where link 50 has a first end 51that is attached to Y portion 33. In this figure, the other end of thelink and the part that it is attached to has been omitted. The link hasa locking head 53 that has a different shape than the locking head 43shown in FIG. 7. Locking head 53 corresponds to the shape of firstcavity 54 that has been machined into the apex of Y-shaped member 33.Other shapes for locking heads and corresponding cavities areenvisioned, and those shown in FIGS. 7 and 8 are for illustrationpurposes to show the interlocking relationship between the links and thecylindrical rings.

In another embodiment, as shown in FIG. 9, link 60 has a first end thatabuts W-shaped portion 32 which again is part of a cylindrical ring (notshown). In this embodiment, locking head 62 has a plurality of teeth 63and a smooth portion shaft 64 that are formed as a part of the W-shapedportion 32. The polymer link either is injection molded to surround orencapsulate the locking head or the portion surrounding the locking headcan be heated and the polymer link pushed over the teeth and shaftportion of the locking head as the polymer softens from the heatedlocking head 62. As the structure cools, the polymer link 60 becomessecurely fastened to the locking head. In this embodiment, the link tip65 abuts the curved portion 66 of the W-shaped portion 32. The link 60can be used to attach any of the U-shaped, W-shaped, or Y-shapedportions 31,32,33.

In another embodiment shown in FIG. 10, two Y-shaped portions 33 areconnected by link 70. The link has a first end 71 and a second end 72which abuts the peaks of the Y-shaped portions. First end 71 is attachedto first locking head 73 and second end 72 is attached to second lockinghead 74. Each of the locking heads have teeth 75 for gripping thepolymer link and a shaft 76 attached to the respective Y-shaped portions33. Link tip 77 is intended to abut Y-shaped portion 33. As describedfor the embodiment of FIG. 9, the polymer link 70 can be injectionmolded as will be further described herein or the locking heads may beheated and the polymer link pushed onto the locking heads andsubsequently cooled to form the attachment.

In another embodiment shown in FIG. 11, link 80 is attached to and abutsa portion of W-shaped portion 32 of a cylindrical ring (again notshown). In this embodiment, the link has a first end 81 and a lockinghead 83 that conforms to the shape of first cavity 84 that is formed inW-shaped portion 32. This embodiment differs slightly from that shownin, for example, FIG. 7, in that in this embodiment the link 80 has alink tip 86 that abuts the inner curve surface of the W-shaped portion.As previously described, injection molding the polymer link 80 is onemethod of forming the attachment and the locking head 83 to attachadjacent cylindrical rings.

In another embodiment of the invention, as shown in FIG. 12, the polymerlink is attached to a W portion 32 of a cylindrical ring by use of anadhesive. More particularly, link 90 has a first end 91 that isadhesively bonded to the W-shaped portion. A first locking head 93 isassociated with the link and a second locking head 94 is associated withthe W-shaped portion. The first locking head corresponds to a firstcavity 95 formed in the link 90, while the second locking head 94corresponds to a second cavity 96 formed in the W-shaped portion. Theadhesive 97 can be any biocompatible adhesive that is well known, suchas a cyanoacrylite-based adhesive. Several adhesives can be usedincluding Locitite 401, 1-06FL, and M-11FL, the latter two of which areurethane-based adhesives. Other adhesives can be used without departingfrom the spirit and scope of the invention. As can be seen in FIG. 12,the adhesive 97 forms the bond for attaching the link 90 to the W-shapedportion 32. Other shapes for the first locking head and the secondlocking head 93,94 can be used in order to enhance the attachment forcebetween the link and the W-shaped portion. Alternatively, link 90 can beattached to the metal ring by a lap joint (with the polymer linkoverlying the connection site) or a butt joint (with the end of thepolymer link adhered to the edge of the metal ring).

In another embodiment illustrating the attachment of the link to thecylindrical ring, as shown in FIGS. 13 and 14, the link is heat-stakedand wrapped around a portion of the cylindrical ring. More specifically,link 100 has a first end and an aperture 102 adjacent to first end. Astake 103 (or peg) is formed of a similar polymer material to that usedfor the links. The stake is positioned a short distance from the firstend and is sized to create and interference with the aperture. In orderto attach the link to a portion of the cylindrical ring, the first endof the link is wrapped around a portion of the cylindrical ring (notshown) in the direction of arrow 105 in order to form a loop 104. Thestake is pushed through the aperture and should have a tightinterference fit to form a locking relationship, or can be heated toflow the aperture and stake to form a more secure attachment.

As previously described, the links of the various embodiments of theinvention are formed from a polymer material then attached in the mannerdescribed. With respect to the embodiments depicted in FIGS. 7-11, thelinks can be injection molded to fill the various cavities described toform the locking heads.

In keeping with one method of the invention for forming the links andattaching them to the cylindrical rings, an injection molding apparatusis shown in FIGS. 15-19. In keeping with the invention, a mandrel 110 isprovided with grooves that correspond to the pattern of the cylindricalrings 12. The cylindrical rings are placed over the mandrel and fittedinto the ring grooves 111. The mandrel also has link grooves 112 inwhich the injected polymer will flow in order to attach one cylindricalring to an adjacent cylindrical ring. After the cylindrical rings arefitted into the ring grooves 111, and as shown in FIG. 16, a pluralityof outer mold covers 115 are fitted around the mandrel and locked inplace by known means, such as by clamping. The outer mold covers 115typically are in cylindrical sections as depicted in FIGS. 16-19 and itis preferred that from two to four arc sections of outer mold covers beused to encase the mandrel 110. Each of the outer mold covers hasgrooves that correspond to grooves in the mandrel. Specifically, theouter mold covers have ring grooves 116 and link grooves 117 thatcorrespond to the ring grooves 111 and link grooves 112 of the mandrel110. The polymer used to form the links is injected by known techniquesthrough gates 118 located at multiple positions along the outer moldcovers. The gates provide openings or apertures through the outer moldcoves to correspond to the location of the link grooves 112,117 so thatas the polymer is injected through the outer mold cover, it will flowinto the link groove 112,117 and form the link pattern.

After the outer mold covers and mandrel have a chance to cool so thatthe polymer solidifies, the outer mold covers 115 can be removed fromthe mandrel 110 and any excess flashing from the gates 118 can beremoved by known means. The cylindrical rings 12 are then removed fromthe mandrel along with the links so that a completed stent with therings attached to each other are formed.

In an alternative embodiment, as shown in FIG. 20, the same mandrel 110and outer mold covers 115 can be used to form polymer links to attachcylindrical rings that add varying degrees of thickness along portionsof the cylindrical ring. For example, as shown in FIG. 20, a U-shapedportion 120 has a thinner portion 121 at the apex 122 and thickerportion 123 as you move away from the apex. In this configuration, oncethe cylindrical ring is mounted onto the mandrel, the outer mold covers115 will require ring grooves 116 that correspond to the thinner andthicker portions 121,123 of the rings. Thereafter, the polymer injectionprocess previously described to form the links is used to form link 124which flows over the thinner portion 121 to connect one cylindrical ringto an adjacent cylindrical ring. In this embodiment, the polymer link124 will encompass or flow around the U-shaped portion 120 at the apex122 to form the attachment of the link to the cylindrical ring. Again,after the assembly has cooled and the polymer has solidified, the outermold covers are removed and the stent is removed from the mandrel. Anyexcess polymer or flash can be removed by known methods.

With respect to the foregoing description of the polymer injectionprocess, it is desirable that the cylindrical rings be placed on themandrel 110 while the rings are in a somewhat expanded configuration. Itis possible, however, to perform the injection mold process when therings are in an unexpanded configuration on the mandrel, but it iseasier in the expanded condition.

The link-to-ring attachment as shown in FIG. 12 can be accomplished bythe mold injection process as described, only an adhesive is added atthe end of the molding process to complete the attachment. For example,referring to FIG. 12 and FIGS. 16-19, the stent rings 12 are placed onthe mandrel 110 as previously described to fit in ring grooves 111. Theouter mold covers 115 are placed over the mandrel so that the mold coverring grooves 116 correspond to the ring grooves 111, and the linkgrooves 117 correspond to the link grooves 112. After the polymermaterial is injected through gate 118, the assembly is allowed to cooland the outer mold covers are removed. Thereafter, the adhesive 97 canbe added to fill first cavity 95 in the link and second cavity in theW-shaped portion 32. After the adhesive solidifies, the assembly isremoved from the mandrel and the stent is formed by the link beingattached to the cylindrical rings.

Similarly, for the link embodiments shown in FIGS. 9 and 10, theinjection molding process is the same as that described for FIGS. 7, 8,and 11. Alternatively, the links 60 and 70 can be heat staked onto thelocking head 62 and first locking head 73 and second locking head 74respectively. For example, the area around the locking heads can beheated so that as the link 60,70 is pushed onto the locking heads, thepolymer material softens and the locking head penetrates the polymermaterial until the link tip 65 and 77 abuts the W-shaped portion 32 andY-shaped portion 33 respectively. After the polymer cools, the teeth 63and 75 assist in attaching the link to the cylindrical ring.

With respect to all of the aforedescribed embodiments in which polymerlinks are used to connect adjacent rings, one or more metal links may berequired between adjacent rings to provide better relative orientationbetween the rings. Also, the metal links will provide more structuralsupport during delivery and after the stent has been expanded andimplanted in the artery or other vessel. Thus, it is in keeping with theinvention that both polymer links and metal links may be used in any ofthe stent embodiments disclosed without departing from the invention.

One method of making the stent 10 of the invention is to first laser cutthe cylindrical rings 12 from a tube so that the rings are not connectedby the aforedescribed polymer links 13. The rings are then placed on amandrel into stent-patterned grooves and encased with a locking sleevehaving a mirror of the stent pattern cut into its inner surface. Theonly exposed region of the stent is the channels that correspond to thelinks that will connect the rings. The mandrel and the encapsulatingsleeve permit the injection of a polymer which fills the channelscorresponding to the links. The polymer is used to form the links whichconnect adjacent rings. The stent forming processes are described inmore detail with the description of the formation of the stentcylindrical rings 12 by a laser cutting process.

The afordescribed illustrative stent 10 and similar stent structures canbe made in many ways. One method of making the stent rings 12 is to cuta thin-walled tubular member, such as stainless steel tubing to removeportions of the tubing in the desired pattern for the stent, leavingrelatively untouched the portions of the metallic tubing which are toform the rings. In accordance with the invention, it is preferred to cutthe tubing in the desired pattern using a machine-controlled laser asillustrated schematically in FIG. 6.

The tubing may be made of suitable biocompatible material such asstainless steel, cobalt-chrome (CoCn, NP35N), titanium, tantalum,nickel-titanium (NiTi), and similar alloys. The stainless steel tube maybe Alloy type: 316LSS, Special Chemistry per ASTM F138-92 or ASTMF139-92 grade 2. Special Chemistry of type 316: per ASTM F 138-92 orASTM F139-92 Stainless Steel for Surgical Implants in weight percent.

Carbon (C) 0.03% max. Manganese (Mn) 2.00% max. Phosphorous (P) 0.025%max. Sulphur (S) 0.010% max. Silicon (Si) 0.75% max. Chromium (Cr)17.00-19.00% Nickel (Ni) 13.00-15.50% Molybdenum (Mo)  2.00-3.00%Nitrogen (N) 0.10% max. Copper (Cu) 0.50% max. Iron (Fe) Balance

The stent diameter is very small, so the tubing from which it is mademust necessarily also have a small diameter. Typically the stent has anouter diameter on the order of about 0.06 inch in the unexpandedcondition, the same outer diameter of the tubing from which it is made,and can be expanded to an outer diameter of 0.1 inch or more. The wallthickness of the tubing is about 0.003 inch.

The tubing is put in a rotatable collet fixture of a machine-controlledapparatus for positioning the tubing relative to a laser. According tomachine-encoded instructions, the tubing is rotated and movedlongitudinally relative to the laser which is also machine-controlled.The laser selectively removes the material from the tubing by ablationand a pattern is cut into the tube. The tube is therefore cut into thediscrete pattern of the finished cylindrical rings.

Cutting a fine structure (0.0035 inch web width) requires minimal heatinput and the ability to manipulate the tube with precision. It is alsonecessary to support the tube yet not allow the stent structure todistort during the cutting operation. In one embodiment, the tubes aremade of stainless steel with an outside diameter of 0.060 inch to 0.095inch and a wall thickness of 0.002 inch to 0.004 inch. These tubes arefixtured under a laser and positioned utilizing a CNC to generate a veryintricate and precise pattern. Due to the thin wall and the smallgeometry of the stent pattern (0.0035 inch typical strut or ring width),it is necessary to have very precise control of the laser, its powerlevel, the focused spot size, and the precise positioning of the lasercutting path.

In order to minimize the heat input into the stent structure, whichprevents thermal distortion, uncontrolled bum out of the metal, andmetallurgical damage due to excessive heat, and thereby produce a smoothdebris free cut, a Q-switched Nd/YAG, typically available fromQuantronix of Hauppauge, N.Y., that is frequency doubled to produce agreen beam at 532 nanometers is utilized. Q-switching produces veryshort pulses (<100 nS) of high peak powers (kilowatts), low energy perpulse (≦3 mJ), at high pulse rates (up to 40 kHz). The frequencydoubling of the beam from 1.06 microns to 0.532 microns allows the beamto be focused to a spot size that is 2 times smaller, thereforeincreasing the power density by a factor of 4 times. With all of theseparameters, it is possible to make smooth, narrow cuts in the stainlesstubes in very fine geometries without damaging the narrow struts thatmake up to stent structure. Hence, the system makes it possible toadjust the laser parameters to cut narrow kerf width which will minimizethe heat input into the material.

The positioning of the tubular structure requires the use of precisionCNC equipment such as that manufactured and sold by Anorad Corporation.In addition, a unique rotary mechanism has been provided that allows thecomputer program to be written as if the pattern were being cut from aflat sheet. This allows both circular and linear interpolation to beutilized in programming. Since the finished structure of the stent isvery small, a precision drive mechanism is required that supports anddrives both ends of the tubular structure as it is cut. Since both endsare driven, they must be aligned and precisely synchronized, otherwisethe stent structure would twist and distort as it is being cut.

The optical system which expands the original laser beam, delivers thebeam through a viewing head and focuses the beam onto the surface of thetube, incorporates a coaxial gas jet and nozzle that helps to removedebris from the kerf and cools the region where the beam interacts withthe material as the beam cuts and vaporizes the metal. It is alsonecessary to block the beam as it cuts through the top surface of thetube and prevent the beam, along with the molten metal and debris fromthe cut, from impinging on the opposite surface of the tube.

In addition to the laser and the CNC positioning equipment, the opticaldelivery system includes a beam expander to increase the laser beamdiameter, a circular polarizer, typically in the form of a quarter waveplate, to eliminate polarization effects in metal cutting, provisionsfor a spatial filter, a binocular viewing head and focusing lens, and acoaxial gas jet that provides for the introduction of a gas stream thatsurrounds the focused beam and is directed along the beam axis. Thecoaxial gas jet nozzle (0.018 inch I.D.) is centered around the focusedbeam with approximately 0.010 inch between the tip of the nozzle and thetubing. The jet is pressurized with oxygen at 20 psi and is directed atthe tube with the focused laser beam exiting the tip of the nozzle(0.018 inch dia.) The oxygen reacts with the metal to assist in thecutting process very similar to oxyacetylene cutting. The focused laserbeam acts as an ignition source and controls the reaction of the oxygenwith the metal. In this manner, it is possible to cut the material witha very fine kerf with precision. In order to prevent burning by the beamand/or molten slag on the far wall of the tube I.D., a stainless steelmandrel (approx. 0.034 inch dia.) is placed inside the tube and isallowed to roll on the bottom of the tube as the pattern is cut. Thisacts as a beam/debris block protecting the far wall I.D.

Alternatively, this may be accomplished by inserting a second tubeinside the stent tube which has an opening to trap the excess energy inthe beam which is transmitted through the kerf along which collectingthe debris that is ejected from the laser cut kerf. A vacuum or positivepressure can be placed in this shielding tube to remove the collectionof debris.

Another technique that could be utilized to remove the debris from thekerf and cool the surrounding material would be to use the inner beamblocking tube as an internal gas jet. By sealing one end of the tube andmaking a small hole in the side and placing it directly under thefocused laser beam, gas pressure could be applied creating a small jetthat would force the debris out of the laser cut kerf from the insideout. This would eliminate any debris from forming or collecting on theinside of the stent structure. It would place all the debris on theoutside. With the use of special protective coatings, the resultantdebris can be easily removed.

In most cases, the gas utilized in the jets may be reactive ornon-reactive (inert). In the case of reactive gas, oxygen or compressedair is used. Compressed air is used in this application since it offersmore control of the material removed and reduces the thermal effects ofthe material itself. Inert gas such as argon, helium, or nitrogen can beused to eliminate any oxidation of the cut material. The result is a cutedge with no oxidation, but there is usually a tail of molten materialthat collects along the exit side of the gas jet that must bemechanically or chemically removed after the cutting operation.

The cutting process utilizing oxygen with the finely focused green beamresults in a very narrow kerf (approx. 0.0005 inch) with the molten slagre-solidifying along the cut. This traps the cut out scrap of thepattern requiring further processing. In order to remove the slag debrisfrom the cut allowing the scrap to be removed from the remaining stentpattern, it is necessary to soak the cut tube in a solution of HCL forapproximately 8 minutes at a temperature of approximately 55° C. Beforeit is soaked, the tube is placed in a bath of alcohol/water solution andultrasonically cleaned for approximately 1 minute to remove the loosedebris left from the cutting operation. After soaking, the tube is thenultrasonically cleaned in the heated HCL for 1-4 minutes depending uponthe wall thickness. To prevent cracking/breaking of the struts attachedto the material left at the two ends of the stent pattern due toharmonic oscillations induced by the ultrasonic cleaner, a mandrel isplaced down the center of the tube during the cleaning/scrap removalprocess. At completion of this process, the stent structures are rinsedin water. They are now ready for electropolishing.

The stent rings are preferably electrochemically polished in an acidicaqueous solution such as a solution of ELECTRO-GLO #300, sold by theELECTRO-GLO Co., Inc. in Chicago, Ill., which is a mixture of sulfuricacid, carboxylic acids, phosphates, corrosion inhibitors and abiodegradable surface active agent. The bath temperature is maintainedat about 110-135° F. and the current density is about 0.4 to about 1.5amps per in.². Cathode to anode area should be at least about two toone.

Direct laser cutting produces edges which are essentially perpendicularto the axis of the laser cutting beam, in contrast with chemical etchingand the like which produce pattern edges which are angled. Hence, thelaser cutting process essentially provides strut cross-sections, fromcut-to-cut, which are square or rectangular, rather than trapezoidal.

The foregoing laser cutting process to form the cylindrical rings 12 canbe used with other metals including cobalt-chrome, titanium, tantalum,nickel-titanium, and other biocompatible metals suitable for use inhumans, and typically used for intravascular stents. Further, while theformation of the cylindrical rings is described in detail, otherprocesses of forming the rings are possible and are known in the art,such as by using chemical etching, electronic discharge machining,stamping, and other processes.

Generally speaking, links 13 can be formed by injection molding by themethods described herein. Some examples of materials that can be used toform the links include polyurethanes, polyetherurethanes,polyesterurethanes, silicone, thermoplastic elastomer (C-flex),polyether-amide thermoplastic elastomer (Pebax), fluoroelastomers,fluorosilicone elastomer, styrene-butadiene rubber, butadiene-styrenerubber, polyisoprene, neoprene (polychloroprene), ethylene-propyleneelastomer, chlorosulfonated polyethylene elastomer, butyl rubber,polysulfide elastomer, polyacrylate elastomer, nitrile, rubber, a familyof elastomers composed of styrene, ethylene, propylene, aliphaticpolycarbonate polyurethane, polymers augmented with antioxidents,polymers augmented with image enhancing materials (e.g., bariumsulfate), polymers having a proton (H+) core, polymers augmented withprotons (H+), butadiene and isoprene (Kraton) and polyesterthermoplastic elastomer (Hytrel).

While the invention has been described in connection with certaindisclosed embodiments, it is not intended to limit the scope of theinvention to the particular forms set forth, but, on the contrary it isintended to cover all such alternatives, modifications, and equivalentsas may be included in the spirit and scope of the invention as definedby the appended claims.

What is claimed:
 1. An intravascular stent, comprising: a plurality of flexible cylindrical rings being expandable in a radial direction, each of the rings having a first delivery diameter and a second implanted diameter and being aligned on a common longitudinal axis; each of the rings being formed of a first material; a plurality of flexible links, each of the links having sufficient column strength to axially separate the cylindrical rings; wherein the first material is a metallic material; at least one of the links being formed of a second material more flexible than the first material; wherein the second material is a polymeric material: each of the links having a first end and a second end, the first and second ends being coupled to adjacent cylindrical rings to couple adjacent cylindrical rings together; and at least one link being coupled between each pair of adjacent cylindrical rings.
 2. A process for providing an intravascular stent, comprising: providing a plurality of flexible cylindrical rings being expandable in a radial direction, each of the rings having a first delivery diameter and a second implanted diameter; the flexible cylindrical rings being aligned on a common longitudinal axis; at least one of the flexible cylindrical rings being formed of a metallic material; providing a plurality of flexible links, each of the links having a first end and a second end, the first and second ends being coupled to adjacent flexible cylindrical rings to couple adjacent flexible cylindrical rings together and each of the links having sufficient column strength to axially separate the flexible cylindrical rings; at least one of the links being formed of a polymeric material that is more flexible than the metallic material; and at least one link being coupled between each pair of adjacent flexible cylindrical rings. 